Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods571Figure 10.30 Examples of sample cells for UV/Vis spectroscopy. From left to right (with path lengths in parentheses):rectangular plastic cuvette (10.0 mm), rectangular quartz cuvette (5.000 mm), rectangular quartzcuvette (1.000 mm), cylindrical quartz cuvette (10.00 mm), cylindrical quartz cuvette (100.0 mm). Cells oftenare available as a matched pair, which is important when using a double-beam instrument.are useful when analyzing a very dilute solution, or for gas samples. Thehighest quality cells allow the radiation to strike a flat surface at a 90 o angle,minimizing the loss of radiation to reflection. A test tube is often used asa sample cell with simple, single-beam instruments, although differencesin the cell’s pathlength and optical properties add an additional source oferror to the analysis.If we need to monitor an analyte’s concentration over time, it may notbe possible to physically remove samples for analysis. This is often the case,for example, when monitoring industrial production lines or waste lines,when monitoring a patient’s blood, or when monitoring environmentalsystems. With a fiber-optic probe we can analyze samples in situ. An exampleof a remote sensing fiber-optic probe is shown in Figure 10.31. Theprobe consists of two bundles of fiber-optic cable. One bundle transmitsradiation from the source to the probe’s tip, which is designed to allow thesample to flow through the sample cell. Radiation from the source passesthrough the solution and is reflected back by a mirror. The second bundlereflectingmirrorlight inlight outflow cellprobe’s tipFigure 10.31 Example of a fiber-optic probe. Theinset photographs provide a close-up look at theprobe’s flow cell and the reflecting mirror